Chemical mechanical polishing (CMP) is one of many processes used in the fabrication of high density integrated circuits. Chemical mechanical polishing is generally performed by moving a substrate against a polishing material in the presence of a polishing fluid. In many polishing applications, the polishing fluid contains an abrasive slurry to assist in the planarization of the feature side of the substrate that is pressed against the polishing material during processing.
The substrate is generally retained during polishing operations by a polishing head. Conventional polishing heads include a retaining ring bounding a substrate retaining pocket. The substrate may be held in the substrate retaining pocket by stiction to a flexible membrane. The retaining ring prevents the substrate from slipping out from under the polishing head during polishing.
During polishing, the retaining ring is typically pressed against the polishing pad. A pressurizable chamber in the carrier head can control the vertical position of the retaining ring. The retaining ring is typically formed of a wearable material, and as polishing progresses, the bottom surface of the retaining ring is worn away. Consequently, the thickness of the retaining ring can change over the course of processing multiple substrates.
Most CMP systems employ a vertically actuatable transfer mechanism, commonly known as a load cup, to transfer substrates between the polishing head and a blade of a robot. The retaining ring and the load cup can include alignment features so that, as the load cup is raised toward the carrier head, the load cup engages the retaining ring and the substrate is aligned with the pocket in the carrier head.